Octafluorocyclobutane as a dielectric material



May 12, 1959 J. K. WOLFE OCTAFLUOROCYCLOBUTANE As A DIELECTRIC MATERIAL.

Filed Feb. -24, 1955 Pfg, 3.

United .States Patent John K. Wolfe, Burnt Hills, N.Y., assignor to General Electric Company, a corporation of New York Application February 24, 1955, Serial No. 490,367

3 Claims. (Cl. 174-15) This invention relates to the use of octauorocyclobutane, cyclic-QPS (hereafter referred to as cyclic-C4118), in electrical insulation. This invention also relates to electrical apparatus insulated with cyclic-C4F8. s

The use of various insulating materials, both liquid and gaseous, is well known in the electrical industry where the electrical apparatus is enclosed with a dielectric material so that the electrical elements therein are insulated from onev another and from the casing in which they are totally enclosed. Of the two types of insulation, gases possess an advantage over liquids in that less weight of material need be employed in the system. l

Although uorocarbon gases as a class are electronegative in character, this alone does not determine their high dielectric strength. Other characteristics present in each specific uorocarbon gas appear to play an important part and such other characteristics may require higher voltages to be impressed before free electrons in the gas can acquire sufficient energy to initiate processes leading to electrical breakdown. The molecular characteristics of each specific gas may increase the breakdown voltage (i.e. the voltage at which the insulator becomes a conductor) by complicated phenomena involving capture or attachment of free electrons by molecules, loss of their energy, such as by dissociation of a molecule by collisions, etc.

Besides possessing high dielectric strength, insulating gases should possess low boiling points and be as noncorrosive as possible to all components of the electrical system.

In Wilson et al., Journal of Applied Physics 2l, page 203 (1950), it has been shown that peruoro-n-butane, C4121() (hereafter referred to as C4F10), is superior in dielectric strength to the lower perfluorinated hydrocarbons, sulfurhexauoride (SP6) and nitrogen.

I have now discovered that cyclic-C4125 possesses even greater electrical strength than does C4F10. In addition, it possesses a lower boiling point than C4F10, and is lessI corrosive to steel than the latter compound.

Cyclic-CgFg, also known as Du Ponts Freon C-318, is described in Patent 2,384,821-Downing et al. This compound has a molecular weight of 200, a boiling point of 21.2 F., a freezing point of -40.7 F. and a liquid density of 1.5241 at 68 F.

One of the most important properties of an insulating gas is that of being able to absorb as much electrical energy as possible without breakdown. As a general rule the dielectric strength of iluorocarbons will increase with increased molecular weight. It is, therefore, unexpected that cyclic-QE, (molecular Weight 200) should have greater dielectric strength than C4F10 (molecular weight 238).

Another important property is that the insulating gas be as non-corrosive as possible to all parts of the electrical system in which it is used at maximum operating temperatures. Since both copper and steel are the most commonly used metals in electrical systems, the corrosiveness of cyclic-C4138 and CFu, toward these metals was compared. Although both cyclic-C4F8 and CrFm produced only a slight reddening on copper in 36 days at 225 C., steel exposed to cyclic-C4138 was not visibly attacked While the same steel exposed to C4F10 at the same temperature and for the same time was blackened. Thus cyclic-C4118 is superior to C4Fm in respect' to its effect on steel. y

Still another important property desirable in an insulating gas is that it be gaseous at as low a temperature as possible. This is important so that it will not condense and settle to the bottom of the apparatus, thus causing loss of electrical insulation and a high and possibly collapsing external pressure on the encasing material. Furthermore, since lowering the pressure within the insulating system decreases the electrical strength of an insulating gas, the gas condensing at the higher temperature would not only be removed from the area in which it is effective as an insulator but would also lose dielectric strength by a reduction of pressure. Thus, cyclic-C4F8 which has a boiling point of 21.2 F. is superior in this respect to C4F10 whose boiling point is 29 F.

The features of this invention may best be understood by reference to the accompanying drawings.

In Fig. 1 there is shown a transformer having a core piece, a plurality of inductively related electrical windings on the core piece, a gas-tight housing encasing the winding and core piece and gaseous cyclic-C4F8 in the housing surrounding the winding and the core piece and filling the interior of the transformer.

The transformer comprises a gas-tight housing or tank 3 tted with a removable cover 7 provided with a pair of external terminals 4 and 5. Extending vertically with respect to tank 3 are a plurality of cooling conduits 8 and 9 through which the gas circulates. Positioned on the interior of the tank 3 is a core piece 1 and a plurality of electrical windings 2 in accordance with conventional transformer construction. The core piece 1 and windings 2 are immersed in cyclic-C4138 which fills the interior of the transformer 6.

Fig. 2 illlustrates a capacitor which is improved by the incorporation therein of the electric insulating material of this invention.

The capacitor, as conventionally shown in Fig. 2, comprises a casing 10 in which are mounted closely spaced sets of electrodes 11, 12 which are supported from a cover plate 17. The electrodes may be maintained in proper operative relation by spacers (not shown) comprising suitable insulating material. The respective spaced sets of capacitor plates are connected respectively to external terminals 13, 14 which are provided with suitable insulators 15, 16. A dielectric material cornprising cyclic-(241:8 fills the interior of the capacitor 18.

The advantages of my invention are not limited to the electrical devices illustrated but may be obtained in other electrical devices, such as switches, X-ray tube housings, bushings, gas-filled cables, etc. Although it is preferred that the cyclic-C4138 be in the vapor phase in some cases the presence of some liquid phase in the container may be of some advantage for it assists heat transfer by its evaporation and recondensation and constitutes a reservoir or source of gas.

Cyclic-C411, is particularly useful as an insulation composition in electrical apparatus such as transformers, capacitors, etc., operated in the vicinity of atmospheric pressure where high voltages are impressed on the electrical apparatus containing closely spaced electrical components.

In Fig. 3 the unbroken lines A and B represent the 1.5 X4irs. (1.5 40;rs. denotes a voltage surge which rises to its peak value in 1.5 microseconds after its incidence and falls to V2 of this peak in 40 microseconds after incidence) impulse breakdown voltage in kilovolts 'I ,Passeurs-y *12, 1959" (kv.) of a 3 inch. diameter parallel plane gaplinv cyclic- C4178 (A)l and C412, (B) at various electrode spacings and at atmospheric pressure. Under these conditions the electric eld is essentially uniform. The abscissa represents electrode spacing in inches and the ordinate represents the breakdown kv. The advantage of cyclic- C4F8 over C4131() is' evident from these lines. For example, at an electrode spacing of 0.6 inch the breakdown liv. is 160 for cyclic-QPS and 140 for C4120, a 14% improvement.

In Fig. 3 the broken `lines A and B represent the average sixty-cycle breakdown voltage in kv.s (root mean square) of 1A" square rod-to-plane gap in cyclic- C4138 (A') and C4F10 (B) at various electrode spacings and at atmospheric pressure. Under these conditions the electric field is essentially non-uniform. The abscissa represents electrode spacing in inches and the ordinate represents the root means square average breakdown voltage. The advantage of cyclic-Clia over C4F10 is evident from these lines. For example, at an electrode spacing of`0.75 inch the breakdown kv. is 87 for cyclic- C4F8 and 81 for C4F10 an improvement of about 8%.

Thus, cyclic C4F8 is superior as an electrical insulation gas to other r'luorocarbons of which C4F10 is representative. it not only possesses a lower boiling point than C4121@ but is also less corrosive to steel than this compound all three properties of which point up the superiority of cyclic-C4F8 as an electrical insulation gas in comparison to CFm.

In addition to its superior electrical properties 25 What Iclaim as newand desire to secure by Letters Patent of the United States is:

l. Anv electric apparatusv comprising the' combination of a closed, gas-tight container, an electrical device therein, having a plurality of conducting members which are adapted to have electrical potentials developed thereacross and between which electrical discharges may occur, and an insulating medium for said device which comprises octailuorocyclobutane.

2L An electrical transformer comprising the combination of a closed, gas-tight container, inductively related electrical windings therein, which are adapted to have.

charges may occur, and interposed dielectric material,v

said dielectric material comprising octauorocyclobutane.

References Cited in the le of this patent UNITED STATES PATENTS 2,384,821 Downing Sept. 18, 1945 2,394,581 Benning Feb. 12, 1946 2,643,282 Greene June 23, 1953 FOREIGN PATENTS 525,244 Great Britain Aug. 23, 1940" 

1. AN ELECTRIC APPARATUS COMPRISING THE COMBINATION OF A CLOSED, GAS-TIGHT CONTAINER, AN AELECTRICAL DEVICE THEREIN, HAVING A PLURALITY OF CONDUCTING MEMBERS WHICH ARE ADAPTED TO HAVE ELECTRICAL POTENTIALS DEVELOPED THEREACROSS AND BETWEEN WHICH ELECTRICAL DISCHARGES MAY OCCUR, AND AN INSULATING MEDIUM FOR SAID DEVICE WHICH COMPRISES OCTAFLUOROCYCLOBUTANE. 